The purposes of this research project continue to be to evaluate the biological effectiveness of the vitamin D metabolites 1,25(OH)2D3 and 24,25(OH)2D3 in humans under normal and pathological conditions--including renal osteodystrophy, bone disease associated with total parenteral nutrition and hypoarathyroidism. A key adjunct to these studies is the availability and continued development and sophistication of steroid competition assays which permit the simultaneous determination in 2.0 ml plasma samples of the concentration of 25(OH)D, 1,25(OH)2D and 24,25(OH)2D.